In a watch, a striking mechanism may be provided to generate a sound or piece of music. To achieve this, the gong of the striking watch or the vibration plate of the musical watch are generally arranged inside the watch case. Thus, the vibrations of the gong or the vibration plate strips are transmitted to the various external parts of the watch. These external parts are, for example, the case middle, the bezel, the crystal and the back cover of the watch case. These large external parts start to radiate sound into the air under the effect of the transmitted vibrations. When a sound is produced either by a gong struck by a hammer, or by the vibration of one or more strips of the vibration plate, these external parts are capable of radiating the produced sound into the air.
Generally, in such a musical or striking watch, acoustic efficiency, based on the complex vibroacoustic transduction of the external parts, is low. In order to improve and increase the acoustic level perceived by the user of the striking or musical watch, the material, geometry and boundary conditions of the external parts must be taken into account. The configurations of these external parts are also dependent upon the aesthetic appearance of the watch and operating stresses, which may limit adaptation possibilities.
The frequency content of the sound from a striking or musical watch must be rich in a frequency range between 1 kHz and 6 kHz. Conventional external parts do not allow efficient radiation in this frequency range. Thus, in order to further improve the vibroacoustic efficiency of the striking mechanism, one or more membranes are arranged inside the watch case. The membranes are dimensioned and configured such that the note or notes produced in the watch case are efficiently radiated. The frequencies of the notes produced must be close to the natural vibration modes of the membranes in order for them to resonate.
Constraints relating to the arrangement of acoustic membranes are generally at variance with the rules of mechanical design for ensuring impermeable sealing, and the mechanical resistance of the watch to shocks and high external pressures. The back cover of the striking or musical watch, which is normally pierced with openings, also requires the connection between the membrane or membranes and the rest of the movement to be sealed. In these conditions, a pierced membrane cannot therefore be used between the back cover and the mechanical movement. Further, a membrane having a too low level of stiffness cannot ensure sufficient resistance to external pressure without risking damage to the movement, which constitutes drawbacks.
EP Patent Application No 1 795 978 A2 discloses a watch, which includes a striking device. This striking device includes two bell-shaped membranes, which are coaxially held in the watch case, one atop the other, by central support pins. There is also provided, between the two bells and the back cover of the watch case, another thin taut membrane secured between the case middle and the pierced back cover of the watch case. By adjusting the radial tension of the other membrane, it is possible to adjust the acoustic radiation frequency of this membrane. However, the two other bell-shaped membranes are not arranged to improve the acoustic level of the sound produced by the striking device, which is a drawback.